Brine dispersal system

ABSTRACT

A desalination brine dispersal apparatus and method employ airlift to remove, oxygenate and disperse brine from a desalination apparatus. The apparatus includes a brine removal conduit having a brine inlet that receives brine from the desalination apparatus, a plurality of brine outlets submerged in seawater and one or more air introduction points located at depths below the brine outlets. The supplied air oxygenates and moves brine through the brine removal conduit and outlets via airlift and disperses the brine into seawater away from the brine removal conduit. The apparatus avoids the formation of concentrated, high shear brine plumes and can disperse brine into seawater over a wide area well away from the brine removal conduit.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.16/484,363 filed Aug. 7, 2019 and entitled “BRINE DISPERSAL SYSTEM,”which is a national stage filing under 35 U.S.C. § 371 of InternationalApplication No. PCT/US2018/017618 filed Feb. 9, 2018 and entitled “BRINEDISPERSAL SYSTEM,” which claims priority to U.S. Provisional ApplicationSer. No. 62/457,034 filed Feb. 9, 2017 and entitled “SUBMERGED REVERSEOSMOSIS SYSTEM,” the disclosures of which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

This invention relates to water desalination.

BACKGROUND ART

The growth of saltwater (e.g., seawater) desalination has been limitedby a number of factors. Such systems typically employ an onshorefacility containing a distillation or evaporation apparatus orseparation membranes (e.g., reverse osmosis or “RO” membranes) suppliedwith seawater from a submerged offshore intake system, and producing aconcentrated brine stream that is returned to the sea. Both the intakeseawater and the concentrated brine stream have high corrosion potentialand consequently require expensive components and equipment. Onshoredesalination facilities typically also require significant amounts ofexpensive seaside real estate. Saltwater desalination has in additionbeen criticized for various environmental impacts, including entrainmentof marine life in the intake water, greenhouse gas production associatedwith producing the energy required, potential harm to marine life causedby the discharged brine, and the use of treatment chemicals that mayenter the ocean.

In the 50 years since the invention of semi-permeable RO membranes,various concepts for submerging such membranes and employing naturalhydrostatic water pressure to help desalinate seawater been proposed.Representative examples include the systems shown in U.S. Pat. No.3,456,802 (Cole), U.S. Pat. No. 4,125,463 (Chenowith), U.S. Pat. No.5,229,005 (Fok et al.), U.S. Pat. No. 5,366,635 (Watkins), U.S. Pat. No.5,914,041 (Chancellor '041), U.S. Pat. No. 5,944,999 (Chancellor '999),U.S. Pat. No. 5,980,751 (Chancellor '751) and U.S. Pat. No. 6,348,148 B1(Bosley), US Patent Application Publication Nos. 2008/0190849 A1 (Vuong)and 2010/0270236 A1 (Scialdone), GB Patent No. 2 068 774 A (Mesple) andInternational Application No WO00/41971 A1 (Gu). An experimental systemis described in Pacenti et al., Submarine seawater reverse osmosisdesalination system, Desalination 126, pp. 213-18 (November, 1999). Itappears however that submerged RO systems (SRO systems) have not beenplaced in widespread use, due in part to factors such as the energy costof pumping the desalinated water to the surface from great depth and thedifficulty of maintaining mechanical moving parts at depth.

For both conventional (viz., onshore, offshore platform-mounted orship-borne) desalination systems and submerged desalination systems, thebrine discharge stream remains an environmental concern. Californiarecently adopted an amendment (the “Desalination Amendment”) to itsWater Quality Control Plan for the Ocean Waters of California to addresseffects associated with the construction and operation of saltwaterdesalination facilities. Included in the Desalination Amendment arerequirements concerning brine discharge. In an effort to meet theserequirements, many desalination facilities have been exploring the useof multiport diffusers (viz., spaced ports or nozzles installed onsubmerged marine outfalls) and the use of ever-higher pressures toinject brine into the surrounding seawater. However, doing so createshigh-shear brine plumes that can harm marine life.

From the foregoing, it will be appreciated that what remains needed inthe art is an improved desalination brine dispersal system featuring oneor more of lower energy cost, lower capital cost, lower operating costor reduced environmental impact. Such systems are disclosed and claimedherein.

SUMMARY OF THE INVENTION

This invention provides in one aspect a wide-area desalination brinedispersal system comprising a brine removal conduit having

-   -   a) a brine inlet that receives brine from a desalination        apparatus;    -   b) a plurality of brine outlets submerged in seawater; and    -   c) one or more air introduction points located at depths below        the brine outlets, for oxygenating and moving brine through the        brine removal conduit and outlets via airlift and dispersing the        brine into seawater away from the brine removal conduit.

The invention provides in another aspect a method for dispersingdesalination brine from a desalination apparatus over a wide area, themethod comprising supplying brine and air to a brine removal conduithaving a brine inlet in fluid communication with the desalinationapparatus, a plurality of brine outlets submerged in seawater and one ormore air introduction points located at depths below the brine outlets;wherein the supplied air oxygenates and moves brine through the brineremoval conduit and outlets via airlift and disperses the brine intoseawater away from the brine removal conduit.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1a and FIG. 1b are graphs illustrating efficiency, capacity andrecommended operating conditions for pumping liquids using an airliftpump;

FIG. 2 depicts various flow regimes overlaid atop a graph of superficialliquid velocity vs. superficial gas velocity for a vertical airlift pumpsystem operated over a range of air and liquid flow rates;

FIG. 3 depicts various flow regimes overlaid atop a graph of superficialliquid velocity vs. superficial gas velocity for a horizontal airliftpump system operated over a range of air and liquid flow rates;

FIG. 4 is a schematic side view of an onshore RO desalination facilityusing the disclosed brine dispersal system;

FIG. 5 is a side perspective view of a portion of the FIG. 4 brineremoval conduit; and

FIG. 6 and FIG. 7 are schematic sectional views of an SRO desalinationsystem using the disclosed brine dispersal system.

Like reference symbols in the various figures of the drawing indicatelike elements. The elements in the drawing are not to scale.

DETAILED DESCRIPTION

The recitation of a numerical range using endpoints includes all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, 5, etc.).

The terms “a,” “an,” “the,” “at least one,” and “one or more” are usedinterchangeably. Thus, for example, an apparatus that contains “a”conduit includes “one or more” such conduits.

The term “air fraction” when used with respect to a two-phase air:liquid(e.g., air:water) flow through a conduit refers to the volumetricfraction, expressed as a percentage, of the air volume over the lengthof the conduit compared to the conduit volume, with the conduit lengthand volume referring to the total length and total volume unlessotherwise specified. Expressed somewhat differently, the air fractionfor such a two-phase flow refers to the air volume as a percent of thetotal volume of air plus liquid in the conduit.

The term “airflow rate” when used with respect to an airlift pumpsupplied by an air compressor refers to the volumetric airflow measuredat the compressor outlet. There are many possible ways to definecompressor operating conditions and specifications (e.g., based onoutlet pressure, flow and temperature). Airflow rates at any given setof conditions and specifications can be converted, using well knownrelationships, to airflow rates at other conditions and specifications.If not otherwise specified herein, airflow rates are measured in cubicfeet per minute at 1 atmosphere (14.73 psi, 1 bar or 100,000 Pascals)and 5-10° F. (41-50° C.). The resulting rates will be numericallysomewhat lower than standard cubic feet per minute (sfcm) ratesdetermined at 70° F. (21° C.), but will be used in recognition of thetypical temperatures that may be encountered at the expected brinedispersal depths.

The term “airlift” when used with respect to a pump refers to a deviceor method for pumping a liquid or slurry by injecting air (andpreferably only by injecting air) into the liquid or slurry.

The term “annular flow” when used with respect to a two-phase flowregime in a conduit refers to a regime in which liquid (e.g., water orbrine) flows primarily as a film along the conduit wall and gas (e.g.,air) flows primarily as a separate phase in the center of the conduit.The gas phase may contain entrained droplets of liquid, in which casethe flow regime may be referred to as “annular flow with droplets” butcan still be regarded as an annular flow regime.

The term “brine” refers to an aqueous solution containing more sodiumchloride than that found in typical saltwater, viz., more than about3.5% sodium chloride.

The term “bubble flow” when used with respect to a two-phase flow regimein a conduit refers to a regime in which gas (e.g., air) primarily flowsas small bubbles within a continuous liquid (e.g., water or brine) phaseflowing through the conduit. The bubbles may be very small, in whichcase the flow regime may be referred to as “dispersed bubble flow” or“finely dispersed bubble flow” but may still be regarded as a bubbleflow regime.

The term “churn flow” when used with respect to a two-phase flow regimein a conduit refers to a regime between slug flow and annular flow inwhich large bubbles of gas (e.g., air), typically having a diameter nearthe diameter of the conduit and a length ranging up to several times thediameter, flow through the conduit in a chaotic and disordered flowpattern along with liquid that may contain numerous small bubbles.

The term “conduit” refers to a pipe or other hollow structure (e.g., abore, channel, duct, hose, line, opening, passage, riser, tube orwellbore) through which a liquid flows during operation of an apparatusemploying such conduit. A conduit may be but need not be linear, and mayfor example have other shapes including branched, coiled or radiatingoutwardly from a central hub.

The term “depth” when used with respect to an airlift pump (or to acomponent of a submerged apparatus) refers to the vertical distance,viz., to the height of a water column, from the free surface of a bodyof water in which the pump or component is submerged to the point ofpump air introduction or to the location of the component.

The terms “desalinated water” and “fresh water” refer to watercontaining less than 0.5 parts per thousand (ppt) dissolved inorganicsalts by weight. Exemplary such salts include sodium chloride, magnesiumsulfate, potassium nitrate, and sodium bicarbonate.

The terms “efficiency” and “efficiency ratio” when used with respect toan airlift pump intended to pump liquids refer to the ratio of the watermass flow rate to the air mass flow rate. When the context indicates,efficiency may refer to the ratio of output pumping power to therequired input power.

The terms “flow regime” or “flow pattern” when used with respect totwo-phase flow from an airlift pump refer to the type and appearance ofbubbles or other airflow along a specified length of the lift conduit.It will be appreciated that at constant airflow rates the flow regimewill vary within any vertical conduit by depth, with the flow regimetypically having fewer or smaller air bubbles at the bottom of theconduit, and at sufficiently high airflow ratios having more or largerbubbles or even an annular column of air as the depth and the associatedhydrostatic pressure in the conduit at that depth both decrease.

The term “lift height” when used with respect to an airlift pump refersto the vertical distance from the water surface to the point ofdischarge. For an airlift pump that discharges above the water surface,the lift height will be positive. For an airlift pump that dischargesbelow the water surface, the lift height will be negative.

The term “maximum capacity” when used with respect to an airlift pump ata given submergence ratio refers to the maximum liquid discharge flowrate attainable with a given system configuration using air as theinjection gas.

The term “maximum efficiency” when used with respect to an airlift pumpat a given submergence ratio refers to the efficiency ratio for a givensystem configuration at which increased energy input provides adiminishing increase in liquid flow rate plotted on the y-axis in atwo-dimensional Cartesian coordinate system (viz., the ordinate) perunit of additional airflow rate plotted on the x-axis (viz., theabscissa). This corresponds to an asymptote for such plot beyond whichthe slope (viz., the ratio of liquid flow rate to airflow rate)diminishes.

The terms “saltwater” and “seawater” refer to water containing more than0.5 ppt dissolved inorganic salts by weight. In oceans, dissolvedinorganic salts typically are measured based on Total Dissolved Solids(TDS), and typically average about 35,000 parts per million (ppm) TDS,though local conditions may result in higher or lower levels ofsalinity.

The term “slug flow” when used with respect to a two-phase flow regimein a conduit refers to a regime in which gas (e.g., air) primarily flowsthrough the conduit as large bubbles, typically having a diameter at ornear the diameter of the conduit and a length ranging from the diameterto several times the diameter, along with a liquid that may containnumerous small additional bubbles.

The term “submerged” means underwater.

The term “submergence” when used with respect to a submerged airliftpump refers to the vertical distance from the water surface to the (oran) air introduction point.

The term “submergence ratio” when used with respect to an airlift pumprefers to the ratio of submergence to lift height.

The term “submersible” mean suitable for use and primarily used whilesubmerged.

The term “superficial velocity” when used with respect to the flow of afluid in a conduit refers to the volumetric flow rate Q (expressed forexample in m³/s) divided by the conduit cross-sectional area A(expressed for example in m²). When used with respect to a two-phaseflow regime (for example, an air:water flow regime), this definition canbe applied to either phase and calculated to provide a hypothetical flowvelocity for a particular phase as if such phase was the only phaseflowing or present in a given cross-sectional area.

The term “two-phase” when used with respect to flowing substances refersto the simultaneous flow of such substances in two different phases,typically as a gas and a liquid.

The term “water flow rate” when used with respect to an airlift pumpthat pumps water refers to the volumetric airflow at the outlet from thepump discharge conduit.

The term “wide area” when used with respect to dispersal of a fluid(e.g., brine) away from a conduit having a plurality of fluid outlets(e.g., brine outlets) distributed along a length of the conduit, meansdispersal into an area, and typically into a volume, encompassing atleast 5 meters of such length. The disclosed area or volume will alsohave other dimensions (e.g., a width, diameter or height) that willdepend in part upon the direction and velocities of fluid streamspassing through the fluid outlets. Because such other dimensions will beaffected by variable factors including fluid flow rates inside andoutside the conduit, and the overall shape of the dispersed fluid plume,the term “wide area” has been defined by focusing merely on the recitedlength along the recited conduit, as such length typically willrepresent a fixed quantity in a given dispersal system.

Airlift pumping systems may be used for a variety of pumping tasks,including not only the pumping of water but also in undersea miningoperations such as dredging the sea floor to recover gold nuggets ormanganese nodules. “Gas lift” is a term commonly used in oil and gasproduction, including offshore and onshore applications, to raisedesired gaseous or oily products to the surface. Airlift and gas liftsystems can also transport solids, e.g., the above-mentioned nuggets andnodules, sand, gold and the like. However, the disclosed brine dispersalsystem will typically involve only the transport of a liquid (viz.,brine) using a gas (viz., air).

Airlift and gas lift systems normally are operated using air or gas andliquid flow rates selected to maximize the amount of desired productobtained per unit of pumping energy expended. For a two-phase systemthat transports air or another gaseous phase and a desired liquidproduct phase, maximum pumping efficiency typically arises when theaverage flow within the conduit carrying the desired liquid product tothe surface represents a so-called “slug” or “churn” flow regime asdiscussed in more detail below. Further details regarding airlift pumpflow regimes may be found for example in Francois et al., A physicallybased model for airlift pumping, Water Resources Research, 32, 8, pp.2383-2399 (1996), Nenes et al., Simulation of Airlift Pumps for DeepWater Wells, Can. J. Chem. Eng., 74, 448-456 (August 1996) and Pougatchet al., Numerical modeling of deep sea air-lift, Ocean Engineering, 35,1173-1182 (2008).

FIG. 1a shows a plot 100 of liquid flow rate Q_(L) versus air flow rateQ_(G) for a vertical airlift system. At a given lift height, there is aminimum air flow value 102 (designated in FIG. 2a as “Q_(Gmin)”) that isrequired to maintain the initial flow of liquid at a steady state rate.As the air flow rate Q_(G) and consequently the volume of air in thedischarge water conduit are increased above Q_(Gmin), the flow of liquidfrom the discharge water conduit and efficiency both initially increase.At an asymptote represented by point 104, the airlift pump efficiency,which corresponds to the slope Q_(G)/Q_(G), reaches a maximum valuedesignated as “Q_(Geff), Q_(Leff)”, and thereafter declines as the airflow rate increases further. FIG. 1b illustrates the pump efficiency ηas a function of the energy input from air introduction, and shows thechange in slope for curve 200 with increasing air flow rate Q_(G). Thepoint of maximum efficiency and the point of maximum capacity occur atdifferent air flow rate values. For airlifting brine, an airlift pumppreferably is operated between the points of maximum efficiency andmaximum capacity. However, operation in other flow regimes may be usedif desired, for example to provide greater or lesser oxygenation of theflowing brine.

The brine removal conduit in the disclosed system may include acombination of vertical, horizontal or oblique sections. The nature ofthe flow regimes that might arise is however most easily understood byprimarily considering the limiting situations represented by verticaland horizontal discharge conduits. FIG. 2 depicts several potentialvertical flow regimes overlaid atop a graph of superficial liquidvelocity vs. superficial gas velocity for a vertical brine airlift pumpsystem operated over a range of air and liquid flow rates. At small airflow rates and at liquid flow rates up to a superficial liquid velocityof about 5 m/s, the system operates in a bubble flow regime in which inwhich air flows as small bubbles dispersed in water. As the air flowrate increases, the bubbles coalesce to form large bubbles that drive a“slug” of liquid up the conduit in a slug flow regime. Further airflowrate increases cause the large bubbles to become unstable and form achurn flow regime. For a vertical conduit pumping liquids, thetransition from maximum efficiency to maximum capacity (see FIG. 1a )occurs in the transition regime between slug flow and churn flow. At yetlarger air flow rates, annular flow regimes arise.

FIG. 3 depicts several potential horizontal flow regimes overlaid atop agraph of superficial liquid velocity vs. superficial gas velocity for ahorizontal brine airlift pump system operated over a range of air andliquid flow rates. At small air flow rates and at liquid flow rates upto a superficial liquid velocity of about 0.1 m/s, the system operatesin a stratified-smooth flow regime. At somewhat higher air flow rates, astratified-wavy flow regime arises. As the water flow rate increasesabove that required to maintain a stratified-smooth flow regime, abubble flow regime eventually arises, with intermittent occurrence ofelongated bubble, slug flow and churn flow regimes. At very high airflow rates and over a relatively wide range of liquid flow rates, anannular flow regime arises.

Air bubbles expand as the depth and hydrostatic pressure decrease. Thusfor a vertical or oblique discharge conduit, the flow regime can varyalong the brine removal conduit length, and may for example representbubble flow at the maximum depth, slug flow or churn flow atintermediate depths and annular flow nearer the surface. In someinstances it may be desirable to employ an annular flow regime over asubstantial portion of the brine removal conduit length, for example toincrease oxygenation, to help force brine through openings in theconduit, or to decrease the average weight of the water:air column andthereby reduce backpressure at the bottom of the brine removal conduit.

The disclosed brine dispersal system may used with any conduit thattransports brine from a desalination apparatus to or into a body ofwater (e.g., to or into a nearby river, lake, sea or ocean) in which thebrine is to be dispersed. Such conduits may carry brine from a varietyof desalination apparatus sources and a variety of desalinationprocesses, including brine produced by onshore distillation orevaporation facilities, brine produced by onshore RO facilities, brineproduced by SRO systems, brine produced at offshore platform-mounted orseaborne (e.g., ship-borne) desalination systems, brine in marineoutfalls, brine in sub-seabed tunnels, and other brine sources andconduits that will be familiar to persons having ordinary skill in theart. The disclosed air supply inserts (e.g., bleeds, pumps, injects orotherwise provides) air into the brine removal conduit at one or morepoints located at or downstream from (viz., distal with respect to) thelowest point in the brine path. The disclosed brine outlets may beupstream or downstream from the first air introduction point, butdesirably are downstream (and more preferably at least 1, at least 2, atleast 5 or at least 10 meters downstream) from such point. In theinterest of overall airlift and dispersal efficiency, the disclosedbrine outlets preferably also are at lesser depths (viz., closer to thesurface) than such air introduction point(s). The term “downstream”consequently may refer to a point having higher elevation than that ofan air introduction point.

FIG. 4 is a schematic side view of one embodiment of an onshore ROfacility 400 that uses the disclosed brine dispersal system. Compressor401 delivers compressed air to high pressure air line 405. Airline 405supplies airlift air to brine removal conduit 407 at air introductionpoint 409. The distal end of brine removal conduit 407 is supported bybuoy 410, which also serves to mark the approximate location of thedistal end of conduit 407. Brine within conduit 407 is dispersed intothe surrounding seawater 411 through brine outlets 413 arrayed along thelength of brine dispersal section 408, which is positioned downstream(viz., above) air introduction point 409.

Pretreatment facility 415 processes seawater drawn in through intake 417and seawater supply conduit 419, and employs filtration elements,settling tanks or other measures (not shown in FIG. 4) to remove organiccontaminants and other solids from the supplied seawater. Filtrant fromfacility 415 passes through conduit 421 into reverse osmosis facility423 where pressure applied to the filtrant forces it into RO membranesto separate the filtrant into fresh water and brine. Brine is sent tosea water 411 via brine removal conduit 407, and freshwater is deliveredto a water processing facility (not shown in FIG. 4) via conduit 427 forfurther treatment such as chlorination, fluoridation orremineralization, and thence to one or more water customers such as amunicipal or rural water system, water reservoir, hotel, resort,agricultural facility or other entity in need of fresh water.

FIG. 5 is a perspective view, partially in phantom, of airline 405,conduit 407 and a portion of brine dispersal section 408. Compressed airtravels within passage 501 past elbow 503 and enters the flowing brinestream 505 at nozzle 507. Bubbles such as bubble 509 carry the flowingbrine stream upwards within brine removal conduit 407 towards brinedispersal section 408. Brine exits conduit 407 through brine outlets 413whereupon it mixes with surrounding seawater 411. Any remaining brineexits conduit 407 at open end 511 and mixes with seawater 411.

FIG. 6 is a schematic view of one embodiment of an SRO desalinationsystem that uses the disclosed brine dispersal system. Further detailsregarding such system are provided in Applicant's copendingInternational Application No. (Attorney Docket No. 4624.01WO01), filedeven date herewith and entitled SUBMERGED REVERSE OSMOSIS SYSTEM, thedisclosure of which is incorporated herein by reference. System 600 issubmerged in saltwater at an appropriate depth between seafloor 602 andsea surface 604. System 600 may if desired rest upon or be anchored toseafloor 602. System 600 is supplied with compressed air via airline orairlines 606 connected to one or more onshore compressors (not shown inFIG. 6). Desalinated water product is removed from system 600 viaproduct water delivery conduit 608. System 600 includes prefilter 610for removal of gross seawater contaminants. Filter 610 may contain oneor an array of any suitable filtration devices, for example membranes,nonwoven webs, woven webs, particles, hollow or solid fibers or otherfiltration structures. System 600 also includes reverse osmosis unit 612containing one or an array of reverse osmosis membranes arranged in apreferred parallel configuration for separation of desalinated water andbrine. In other embodiments, the membranes may be configured in series,or both in series and in parallel. Seawater enters system 600 via inletscreen 611 atop prefilter 610. Airlines 614, 616 and 618 extend fromairline(s) 606 and may be controlled by on-shore valves, orifice platesor (as shown in FIG. 6), by individually actuated valves. Airline 614supplies lift air to product water delivery conduit 608 for use indirecting desalinated water product through delivery conduit 608 viaairlift pumping. Airline 616 supplies purge air to backflush (and ifdesired, via a further suitable valved or otherwise controlled injectionpoint, to flush) prefilter 610. The use of such purge air can remove orprevent the buildup of contaminants and overcome or avoid clogging, andmay be carried out continuously or at any appropriate interval orsequence. Airline 618 supplies airlift to remove concentrated brine fromreverse osmosis unit 612 via brine removal conduit 620, for dispersal atone or more locations remote from system 600, as discussed in moredetail below.

FIG. 7 shows system 600 in greater detail. Rough screen 602 blocks theentry of fish and other large objects into system 600. Coupling 704joins prefilter 610 to reverse osmosis unit 612, and delivers filteredseawater to reverse osmosis unit 612. Air bubbles 708 may be suppliedfrom time to time or continuously beneath prefilter 610 to carry out airpurging as discussed above. Fresh product water exits reverse osmosisunit 612 via collector 706 and enters product water conduit 608,whereupon airlift (supplied from airline 606 via submerged valve 710 andairline 614) can be used to remove the product water. Manifold 716collects brine from reverse osmosis unit 612 and directs it into brineremoval conduit 620. Brine removal conduit 620 is provided with airliftusing air supplied from airline 606 via submerged valve 714 and airline618. Submerged valves 710, 712 and 714 can be used to regulate the flowof air through airlines 614, 616 and 618 into system 600, and may in theinterest of simplicity and reduced maintenance be eliminated andreplaced by onshore valves or other airflow control measures. Brineremoval conduit 620 may if desired be combined with or serve as ananchor or tether for a buoy that indicates the SRO system location. Anydesired flow regime may be used in brine removal conduit 620, forexample a slug, churn or annular flow regime. A substantial portion ofbrine removal conduit 620 beyond (viz., above as shown in FIG. 6 andFIG. 7) airline 618 includes a plurality of perforations or otheropenings 718 in the sidewall of brine removal conduit 620. The openings718 provide brine outlets through which brine can disperse into seawateraway from brine removal conduit 620. Depending on the size, shape,extent and axial orientation of such openings and the flow of brinewithin brine removal conduit 620, seawater may be drawn into some of theopenings 718 in brine removal conduit 620 and thereby provide brinedilution within brine removal conduit 620.

Although not shown in FIG. 6 and FIG. 7, persons having ordinary skillin the art will understand that system 600 may include an electricalsupply and appropriate electronic controls to operate air valves orother submerged components, measure desired operating parameters (e.g.,pressures, temperatures, flow rates and the like), and to handle otherelectrically-driven or electrically operated equipment or othersignaling needs. Preferably however the use of submerged electricalcomponents is minimized or eliminated. The disclosed submerged optionalvalves may for example be operated using air pressure provided via oneor more additional air supply lines, or eliminated altogether bysupplying air at appropriately varied pressures from the air compressorsystems, optionally together with appropriate arrangement of therespective depths at which the disclosed airlines inject air into theprefilter, product water stream or brine stream.

The FIG. 6 and FIG. 7 SRO system preferably lacks submerged moving partsand especially wearing parts (e.g., pump impellers, shafts, valves andother components) that might by design or through the failure of a sealor enclosure come into contact with seawater or brine beyond theirdesigned capability or suitability. In preferred embodiments, thedisclosed SRO system operates entirely without such failure-pronesubmerged parts as pumps, motors and valves, is composed entirely ofseawater-tolerant materials, and provides steady-state, continuous oressentially continuous RO desalination using the hydrostatic weight ofthe ocean above the membrane or membrane assembly to supply the pressurerequired to drive the pure water through the membrane while leaving mostof its salts behind. Maintenance needs can accordingly be reduced byavoiding frictional sliding surfaces, pump cavitation, motor or bearingfailure and other causes of wear or premature component failure. In oneembodiment of the disclosed system, the brine airlift pressure can beset at the same pressure as the discharge airlift pressure by injectingair for the brine airlift at an appropriately higher elevation than forthe discharge airlift. This allows the use of one air pressure lineinstead of two separate lines having different pressures, helps preventthe accidental injection of air into the RO membranes, and providesfurther simplification.

The disclosed brine dispersal system may be used to remove and airliftbrine produced by a variety of types of onshore, offshoreplatform-mounted, seaborne (e.g., ship-borne) or submerged desalinationapparatus, and to disperse it into seawater into two and more preferablyinto three dimensions. In preferred embodiments the brine is dispersedinto one or more substantial vertical portions of a water column orcolumns above or remote from the disclosed SRO apparatus. The reciteddispersion can occur over a wide area or areas at one or more locationsremote from the desalination apparatus. Doing so can avoid the localizeddischarge of concentrated brine dispersed by high-pressure point-sourcediffusers as commonly used to disperse RO brine today, and the possibleharm to marine life from high salinity or diffuser shear forces. Ifdesired, more than one air introduction (e.g., air injection) point maybe employed, with higher introduction points typically requiring lessenergy to operate the associated brine airlift, and lower introductionpoints providing a greater conduit length along which oxygenation,dilution or dispersion may take place. The brine removal conduitpreferably rises vertically or upwardly away from the air introductionpoint(s) and terminates at a lesser depth than the air introductionpoint(s) so as to facilitate airlift of brine within the brine removalconduit. The brine removal conduit preferably has a substantial lengthbeyond the air introduction point, e.g., at least 5 meters, at least 10meters, at least 20 meters, at least 30 meters, at least 50 meters or atleast 100 meters. The brine removal conduit preferably rises verticallyor upwardly away from the air introduction point(s) and terminates at alesser depth than the air introduction point(s) so as to facilitateairlift of brine within the brine removal conduit. The brine removalconduit preferably has a substantial length beyond the air introductionpoint, e.g., at least 5 meters, at least 10 meters, at least 20 meters,at least 30 meters, at least 50 meters, at least 100 meters, at least500 meters or at least 1,000 meters. If desired, the brine removalconduit may divide or subdivide into a plurality of preferablyupwardly-directed arms each of which may carry airlifted brine anddisperse it into the surrounding seawater.

The brine-dispersing portion of the brine removal conduit preferablycontains a plurality of perforations or other openings in the conduitsidewall (or even one-way or other valves if desired) that provide brineoutlets. The brine outlets may be located below and more preferably arelocated above the air introduction point(s). The outlets may dispersebrine at a variety of depths, for example at depths above, below or bothabove and below a thermocline or halocline. The disclosed brine outletsare desirably sized, positioned and oriented to allow the dispersion ofbrine into the surrounding seawater and well away from the brine removalconduit. The brine outlets preferably are arranged over a substantiallength along the conduit (and more preferably are arranged over asubstantial vertical portion of a water column) of at least 5 meters,and in some embodiments, at least 10 meters, at least 20 meters, atleast 30 meters, at least 50 meters, at least 100 meters, at least 500meters or at least 1,000 meters. A variety of brine outlet openingshapes may be employed, including circular holes, slots, polygons,tapered ducts and other shapes. Vanes or other deflectors may bepositioned within the brine removal conduit to add turbulence to or todirect the brine through brine outlets. Brine can also be expelled fromthe brine outlets due to the expansion of rising air within the brineremoval conduit. If desired, some of the disclosed openings may besized, oriented or positioned to allow diluting seawater to be drawninto the moving brine stream within the conduit, e.g., via the Venturieffect, and thereby serve as brine-diluting seawater inlets. Whetherused to expel brine from the conduit or to draw diluting seawater intothe conduit, the disclosed openings may extend along a substantialextent (for example, at least 1%, at least 2%, at least 3%, at least 4%,at least 5%, at least 10%, at least 20%, at least 30% or at least 40%)of the brine removal conduit length beyond the first air introductionpoint. The size, orientation, frequency and positioning of the disclosedopenings may if desired vary along the length of the conduit, and mayfor example represent larger openings at distances close to the firstair introduction point and smaller openings at distances further fromthe desalination apparatus, or vice versa. One or more portions alongthe length of the brine removal conduit after the first air introductionpoint may be free of openings, for example to allow for enhancedoxygenation of moving brine within such portion. The furthest (andpreferably uppermost) end(s) of the brine removal conduit may be open,partially closed, or closed. Preferably there are sufficient brineoutlet openings to disperse the oxygenated brine stream over a largerarea (viz., into a larger volume of seawater) than would be obtainedusing point-source diffusers. In addition, the diffusion flow throughthe brine outlets preferably is not highly pressurized and thus does notcreate shear forces that might harm marine life.

The extent to which the brine is diluted, oxygenated or dispersed may becontrolled or influenced by a number of factors, including the number,size, shape and axial orientation of the disclosed openings, thepressure and volume of introduced brine airlift air, the respectivevelocities of the disclosed brine and brine airlift flows, and thepresence of turbulence at or after the air introduction point(s).

The disclosed combination of an airlift and a brine delivery conduitwith appropriate openings can permit removal, oxygenation, dilution, anddispersal of brine produced by a desalination apparatus over asubstantial area remote from the apparatus. This can for example permitdispersal of brine high above a seafloor, without creating an unsafeenvironmental condition due to brine, which is denser than seawater,pooling on the seafloor. Such pooled brine could harm benthic-dwellingmarine life, for example by causing hyper-saline conditions on the oceanfloor. Use of an airlift pump to disperse the brine can also save energycompared to the use of marine outfall lines or pressurized brinediffusers commonly employed with shore-based RO plants. In addition, thedisclosed airlift brine diffusion system can diffuse brine into a muchlarger area (viz., volume of nearby water) than is the case for typicalmarine outfall lines or pressurized brine diffusers. The disclosedoxygenation can also reduce the incidence of naturally-occurring orotherwise induced hypoxia or dead zones in nearby seawater.

In a further preferred embodiment for use with RO desalination, thevolume or pressure of airlift air supplied to the disclosed brineremoval conduit can be designed, set or adjusted so that during orfollowing startup, the brine airlift air will provide positive controlof the volume of saline water flowing through the RO membranes. This canhelp prevent polarization at the boundary layer near the membranesurface, and will also discourage membrane fouling or scaling. Inaddition, such control can facilitate adjustment of the salinity of thebrine stream, allow modification (e.g., reduction) of the brine streamairlift demand, or allow for sizing or resizing of pretreatmentconditions and capacities. In an especially preferred embodiment, thevolume or pressure of brine airlift air is designed, set or adjusted tooptimize the RO membrane product water recovery rate and membranehealth.

Expressed in terms of the air:brine volumetric ratio (determined shortlyafter the point at which air is injected into the brine removal conduit,and before taking into account the possible entry into the conduit ofseawater dilution streams via the disclosed openings), air:brine ratiosof at least about 1:99, at least about 5:95, at least about 10:90, atleast about 15:85 or at least about 20:80 may be employed. The air:brineratio may under some conditions be as high as about 99:1, as high asabout 95:5, or as high as about 90:10 but under normal operatingconditions typically will be less, for example up to about 60:40 or upto about 50:50.

In comparison to conventional marine outfall lines or multiportdiffusers, the disclosed brine dispersal system can provide improvedbrine dispersal with reduced capital and energy requirements. If thedisclosed brine airlift system is also used to set or adjust the volumeof saline water flowing through the RO membranes, then RO systemperformance can be controlled at much lower capital cost than wouldtypically be required by the variable frequency drives and seawaterpumps typically used to control the pressure and flow rate of seawaterthrough RO membranes in conventional onshore systems.

Suitable compressor equipment for attaining the disclosed airlift isavailable from a variety of sources that will be familiar to personshaving ordinary skill in the art. If desired, compressor units ofdifferent types or having different capacities may be combined with oneanother or with suitable reserve tanks to provide backup, auxiliary orcomplementary compressed air supplies. For example, a seaborne,submerged or surface platform-mounted compressor unit may be employed inaddition to an onshore compressor unit, and any or all of these may ifdesired be powered in whole or in part by energy derived from waves,wind or sunlight.

As a part of the system design or the startup or operation proceduresfor any of the disclosed onshore, platform-mounted, seaborne orsubmerged desalination systems, the brine airlift supply should be set,controlled or adjusted to provide proper brine flow and dispersal, whileminimizing high shear conditions near the brine outlets that might harmmarine life. This may be done for example by including an orifice plate(located at the surface or more preferably submerged) to limit airflowin the brine airlift line, or by using a valve or regulator (alsolocated at the surface or submerged) for airflow control or adjustmentduring startup or operation. The air pressures and air volumes requiredfor startup and continuous operation of the brine airlift will depend inpart on the depth at which air introduction occurs; the shape, size, andinclination of the brine removal conduit; and the shape, size, numberand orientation of the brine outlet openings. Such pressures and volumesmay be estimated or empirically determined. As a starting point, thestartup and operating pressures desirably are each at least about 14.5psi (1 bar) above the hydrostatic pressure at the point of airintroduction, and may be at least about 29 psi (2 bar), at least about44 psi (3 bar), at least about 58 psi (4 bar), at least about 73 psi (5bar) or at least about 145 psi (10 bar) above such hydrostatic pressure.The operating pressure and operating airflow desirably are controlled soas to avoid backflushing the brine removal conduit. For an onshore orsubmerged desalination apparatus employing RO membranes, the operatingpressure desirably avoids backflushing or the creation of highbackpressure at the RO membrane outlet. Fortunately however, operationof the disclosed brine airlift can reduce salinity buildup at such ROmembranes and can increase the membrane product water flow rate oroutput.

Attainment of a desired brine airlift operating condition may dependless on controlling the air pressure than on controlling the ratio ofair to water. The air fraction (as averaged over the brine removalconduit length from the first air introduction point to the first brineoutlet from which brine leaves the conduit) may for example be at leastabout 1%, at least about 5%, at least about 10%, at least about 30% orat least about 40%, with the latter value typically being characteristicof a slug or churn flow regime. Higher air fractions may be employed,and may be more likely to provide an annular flow regime over anappreciable portion of the brine removal conduit, for example an airfraction up to about 60%, up to about 70%, up to about 80% and in someembodiments up to about 90%, 95% or even 99%. Mechanical pumping assistmay be used if needed to attain sufficient brine flow. In a preferredembodiment however, brine removal is accomplished using only thepressure available at the desalination apparatus brine outlet and thedisclosed airlift, and without the need for mechanical pump componentsbetween the desalination apparatus brine outlet and the brine removalconduit air introduction point(s).

Exemplary depths for operation of the disclosed brine dispersal systemare for example from just below the surface (e.g., from about 10 m),from about 100 m, from about 300 m, or from about 500 m, and up to about2,000 m, up to about 1,500 m or up to about 1,000 m. Preferred depthsare from just below the surface to about 1500 m depth. Operation atdepths below the photic zone (depending upon water clarity,corresponding to depths up to about 200 m) is more preferred, asrelatively few marine organisms are found below the photic zone. Biofilmgrowth may also be discouraged by placing the brine outlets at depthshaving no light, low oxygen, and cold water temperatures.

For SRO systems using the above-mentioned Dow FILMTEC RO membranes andairlift without mechanical pump assist to remove product water anddisperse brine, a depth of 680 m or more in preferred in order toprovide sufficient hydrostatic pressure for permeation to take placethrough the membrane at the recommended 800 psi (55 bar) pressuredifferential across the membrane. The chosen depth and pressure valuesmay however vary in RO systems that take advantage of future membranedevelopments enabling or requiring lower or higher differentialpressures or higher or lower membrane backpressures. Adjustments toaccommodate such developments may increase or decrease the preferredoperating depth for the disclosed SRO system. For many membranes, thepressure on the low-pressure side typically will not change appreciablywith depth, and consequently changing the depth of operation may sufficeto adjust the differential pressure across the membrane and achieveoptimal operating conditions. In a preferred SRO embodiment,desalination is driven entirely by hydrostatic seawater pressure on thehigh-pressure side of the RO membranes, a low-pressure condition ismaintained on the outlet or product side of the membranes by a flow ofcompressed air supplied from the surface at a flow rate and pressuresufficient to create an annular flow regime for air and water over asignificant portion of the delivery conduit and adequately evacuate thedesalinated water product, and brine dispersal is carried out using onlyairlift from the same surface air supply. Once such a system is at theproper depth and air is flowing at the correct volume and pressure, thesystem preferably continuously desalinates seawater, delivers pure waterto the surface, and disperses brine, with no moving parts below thewaterline that would be subject to wear or breakage.

In some prior SRO designs, especially those that rely on a pressure pumpto force seawater through the membranes, thick pressure-resistantvessels are employed to contain the high pressures needed for membraneseparation. In preferred embodiments of the disclosed SRO desalinationsystem, the prefiltration elements and RO membranes will not requirepressure-resistant vessels, as they will already be immersed at asufficiently high pressure in the fluid to be purified. Desirably thedisclosed SRO system merely maintains a sufficiently low pressure on themembrane discharge side, and a sufficient inlet side-outlet sidepressure differential, so as to allow proper membrane operation withoutthe use of a surrounding pressure-resistant vessel. The disclosed SROsystem consequently can produce significantly lower concentrations ofsalt in the brine stream than will be the case for conventional RO, asthe elimination of the requirement for pressure vessels permits the ROmembranes to be arrayed in parallel rather than the typical seawaterdesalination industry practice of 5-7 membranes in a serial arrangement.A parallel array eliminates a common failure point in conventional ROsystems, namely the o-ring interconnections between membranes. Aparallel arrangement also permits higher product water production permembrane. In addition, a parallel membrane arrangement creates much lesssalty brine than a train of single membranes operating in series, andthis salinity can be adjusted by adjusting the brine airlift operatingparameters. The disclosed SRO system's ability to achieve low brinesalinity would be beneficial to sea life and would allow easier brinedilution. For example, using seawater containing 35,000 ppm TDS, thedisclosed system may provide brine containing 38,043 ppm TDS (a 9%increase) versus the near-doubling in discharge stream salinity that mayarise using conventional serially-configured onshore RO.

These and other advantages of the disclosed brine dispersal system thusmay include one or more of:

-   -   Reduced power consumption.    -   Reduced greenhouse gas emissions to diffuse a given quantity of        brine.    -   Reduction or elimination of onshore or offshore high pressure        water pumps.    -   Reduced number of parts requiring expensive alloys and other        exotic materials resistant to seawater corrosion.    -   Reduced localized brine emission.    -   Parallel rather than series membrane configurations with even        lower-salinity brine discharge.    -   Increased oxygenation of nearby seawater and reduction in        hypoxia.

Having thus described preferred embodiments of the present invention,those of skill in the art will readily appreciate that the teachingsfound herein may be applied to yet other embodiments within the scope ofthe claims hereto attached. The complete disclosure of all patents,patent documents, and publications are incorporated herein by referenceas if individually incorporated.

1. A wide-area desalination brine dispersal system comprising a brineremoval conduit having: a) a fluid introduction point that receivesbrine from a desalination apparatus submerged in seawater; and b) aplurality of spaced fluid outlets submerged in seawater at lesser depthsthan the fluid introduction point; wherein the brine removal conduitdisperses the brine into seawater away from the brine removal conduit,at a lesser depth than the desalination apparatus, and without causingseafloor pooling of brine.
 2. A system according to claim 1 wherein thebrine removal conduit disperses brine without creating high-shear brineplumes that harm marine life.
 3. A system according to claim 1 whereinthe brine removal conduit disperses brine without creating high-salinitybrine plumes that harm marine life.
 4. A system according to claim 1wherein the brine removal conduit has a length at least 5 meters beyondthe fluid introduction point.
 5. A system according to claim 1 whereinthe brine removal conduit has a length at least 20 meters beyond thefluid introduction point.
 6. A system according to claim 1 wherein thebrine removal conduit has a length at least 50 meters beyond the fluidintroduction point.
 7. A system according to claim 1 wherein the brineremoval conduit disperses brine above a thermocline or halocline in suchseawater.
 8. A system according to claim 1 wherein the desalinationapparatus comprises a distillation or evaporation apparatus.
 9. A systemaccording to claim 1 wherein the desalination apparatus comprisesreverse osmosis membranes.
 10. A system according to claim 9 wherein thedesalination apparatus is at a depth that enables hydrostatic pressureto drive seawater through the membranes.
 11. A system according to claim9 wherein the membranes are not encased in a surroundingpressure-resistant housing.
 12. A system according to claim 1 whereinthe system does not include submerged frictional sliding surfaces, asubmerged mechanical pump or a submerged valve.
 13. A system accordingto claim 1 wherein the brine removal conduit comprises one or more airintroduction points located at depths below the brine outlets; andwherein air supplied to the one or more air introduction pointsoxygenates and moves brine through the brine removal conduit and outletsvia airlift and disperses the brine into seawater away from the brineremoval conduit.
 14. A system according to claim 13 wherein the airliftis provided by an onshore, ship-borne, submerged or surfaceplatform-mounted compressor unit.
 15. A system according to claim 13wherein the airlift is provided by a compressor unit powered at least inpart by energy derived from waves, wind or sunlight.
 16. A systemaccording to claim 13 wherein the airlift drives brine up the brineremoval conduit in a slug or churn flow regime.
 17. A system accordingto claim 13 wherein the brine removal conduit provides increasedoxygenation of nearby seawater and a consequent reduction in hypoxia ordead zones in nearby seawater.
 18. A system according to claim 1 whereinthe brine removal conduit has a first brine outlet from which brineleaves the conduit, a first length of at least one meter between suchfluid introduction point and such first brine outlet, and a secondlength between such first brine outlet and the distal end of the brineremoval conduit of at least 5 meters.
 19. A system according to claim 18wherein the second length is at least 50 meters.
 20. A system accordingto claim 18 wherein the second length includes openings that allowdiluting seawater to be drawn into flowing brine within the brineremoval conduit, and the brine outlets extend along at least 30% of thesecond length.